Update High-level multiplayer documentation for Godot 4.0

[GamesOnAcidzz]

Godot Alpha 6

RPC changes still aren't documented:

https://docs.godotengine.org/en/latest/tutorials/networking/high_level_multiplayer.html?highlight=high%20level

it still uses the remote, master... instead of @rpc(any_peer, call_local)....

[benbot]

@GamesOnAcidzz do you know which PR changed the multiplayer api?

[DeadlyEssence01]

I've started some work on this.

[RadiantUwU]

RPC still not documented

[mhilbrunner]

Fixed by https://github.com/godotengine/godot-docs/pull/7675